In the semiconductor industry, semiconductor logic devices are most commonly manufactured using MOS processing. These devices, which include a wide variety of types and applications, employ logic circuitry to manipulate or store input data and NMOS transistors to provide the manipulated or stored data at an output terminal of the device. The NMOS transistors are typically configured in the form of an output buffer (or driver) with the output terminal providing the data in the form of a logic "1" or a logic "0."
Typically, the logic "1" and logic "0" data output signals correspond respectively to 5-volt and 0-volt signal levels controlled by the output buffer. For example, to provide a logic "0" at the output terminal, the output buffer converts the signal at the output terminal from a logic "0" (or an undefined floating level) to the 0-volt level. Ideally, once the correct logic level is provided at the output terminal, there is no change until the output buffer is commanded to change the logic level.
In a circuit having NMOS output-driver transistors, converting to a logic "0" can be accomplished quickly without using any special biasing circuitry at the inputs of the NMOS transistors. Quickly converting to a logic "1," however, is more difficult and often requires "precharging circuitry" to permit the output buffer to convert its output signal to the logic "1" level within a desired period of time. Precharging circuitry typically couples the input of the output driver, via NMOS transistors, to a positive supply voltage, as soon as it can be determined that the output buffer will be providing a logic "1" signal level.
In certain applications, NMOS output buffers have also been implemented using bootstrapping circuitry which enhances the precharging circuitry by coupling the input of the output driver to a signal level that is greater than the positive supply voltage. This increases the performance of the output buffers and permits the output drivers to provide the logic "1" level for an extended time period before leakage at the input of the output driver erases the logic "1" level.
In connection with the present invention, it has been discovered that the extended time period provided by bootstrapping circuitry, arranged as described above, is unduly limited. An NMOS output buffer, even though it is bootstrapped, will fail to provide the specified logic "1" output level after a substantial period of time. Consequently, this can result in failure of the logic device.
One possible approach for overcoming this problem would be to further increase the level of the signal coupled to the inputs of the output drivers, so that the time period required to cause a device failure would be further extended. This approach would be lacking, however, in that, for many applications, it still imposes an unacceptably limited time period before device-failure results.
Accordingly, there is a need for a device having a data output circuit which overcomes the above deficiencies of the prior art.